ffmpeg -vsync 2 -i video-0.mkv -acodec libopus -i audio-0.mkv -acodec libopus -i audio-1.mkv -vf scale=w=640:h=480:force_original_aspect_ratio=decrease -filter_complex "[1:a]aformat=sample_fmts=fltp:sample_rates=48000:channel_layouts=stereo,aresample=async=50[1:a];[2:a]aformat=sample_fmts=fltp:sample_rates=48000:channel_layouts=stereo,aresample=async=50[2:a];[1:a][2:a]amerge=inputs=2[mergedAudio]" -shortest -c:v h264 -c:a libopus -b:v 1024k -b:a 128k -map 0:v -map [mergedAudio] -r 25  -preset veryfast -crf 23 -y out.mkv